CS491 Great Principles
Assignment A8
Due 11/4/02


For the next time, we'll talk about the principles of
performance measurement and prediction of computer systems and
networks.  Everyone is interested in how long it will take to
run a computation on a given configuration of hardware and in
the presence of contention from other jobs.  In preparation,
read the queueing network tutorial linked to the CS471 web
resources section (it is a reprint of a pair of American
Scientist essays I wrote) and also the queueing module of the
ArtOS book (cne.gmu.edu/pjd/ArtOS).  Write short answers to
these questions:

(1) Why is it a concern to know how long it will take a system
to respond to a user's request?  And how many user jobs are
completed every hour?

(2) Flow balance is an assumption used in most performance
analyses.  What is flow balance?  How much error can be
introduced by assuming flow balance for a model of a system that
is not flow balanced?

(3) Under what conditions is it true that the bottleneck of a
system is the server with the largest demand?  How much error
would be introduced by assuming this for a real system?

(4) In the MVA algorithm, the first equation says that response
time at a service can be computed as the service time multipled
by a stretch factor equal to 1 plus the queue length observed by
the new arrival to the server.  That queue length is computed as
the overall average queue length when there is one less user in
the system.  The assumption that the arriving user sees the same
average size queue as the outside observer with one less user in
the system is called homogeneity.  Homogeneity asserts that the
arriving user acts as an outside observer in a system where that
user is excluded from the system population.  Homogeneity is
sometimes phrased as "the off line behavior of a server is the
same as its on-line behavior."  Comment on how this latter
definition equates to the former.  What kinds of errors might be
introduced by assuming that the servers are homogeneous in this
way?  If the real system really is homogeneous, would the MVA
equations compute the measured values of throughput and response
time?